The simple solution I have used for making interconnects is wrap each individual wire with Teflon Thread Sealing Tape, basically, that leaves you free to use UPOCC silver or any other choice of wire. Then you make the litz wire combining the individual strands. I have done this for internal tone-arm wire and for cartridge leads. The only disadvantage is losing the air dialectic advantage of cotton or teflon oversized tubing. It is a lot of work. I normally work with cotton sleeving, using lots of 20 or 28 AWG wires, individually in cotton and then a very large cotton sleeve to put them all through. With 28 AWG wire I use a sewing needle to get them through the cotton sleeves. The cotton sleaves I get dirt cheap through our local haberdashery shop.
Cheers,
Jaap